In general, a wireless communication apparatus uses a mixer circuit for frequency conversion to a RF (Radio Frequency) signal for communication, which is a relatively high frequency, from an IF (Intermediate Frequency) signal for signal processing, which is a relatively low frequency, or frequency conversion to an IF signal from a RF signal.
FIG. 1 is a circuit diagram showing a structure of a single balance-type mixer circuit that is used in a wireless communication apparatus and the like.
As shown in FIG. 1, a single balance-type mixer circuit has two mixer elements 51 and 180-degree phase combination circuit 52. Mixer elements 51 mix two IF signals of reverse-phase, which are differential signals, and two local oscillation signals of in-phase (hereinafter, referred to as LO signals) and output an upper sideband signal and a lower sideband signal that are necessary for communication. 180-degree phase combination circuit 52 combines the two inputted signals so that they have a phase difference of 180 degrees and outputs a signal after the combination. Hence, the upper sideband signal and lower sideband signal generated from mixer elements 51 are combined to be in-phase by 180-degree phase combination circuit 52 and are then outputted as a RF signal that is used in the communication.
At this time, although the LO signals, which are unnecessary for communication, are outputted from mixer elements 51, the two LO signals inputted in-phase to mixer elements 51 are outputted in-phase without change. Thus, the LO signals are combined to become reverse-phase by 180-degree phase combination circuit 52, so that they are cancelled and removed.
In the meantime, 180-degree phase combination circuit 52 shown in FIG. 1 can be used as a 180-degree phase splitter when a signal is inputted from the output port (Output) thereof and when it is taken out from the input ports 0, 180. In this case, it is possible to obtain two IF signals having a phase difference of 180 degrees by inputting a RF signal and a LO signal to the mixer elements. The circuit that splits or combines the signals to have a phase difference of 180 degrees is used as a circuit that converts a differential signal into a non-differential signal or a non-differential signal into a differential signal, a circuit that splits a differential signal to a plurality of active elements, a circuit that combines a differential signal, and the like. Due to this, there has been a recent increase in demand such that the 180-degree phase combination circuit (180-degree phase splitting circuit) should be used in a microwave IC that is used in a wireless communication apparatus and the like. Meanwhile, in the microwave IC, a CPW (Coplanar Waveguide) line is used as a transmission line because the processing of the underside surface of a substrate is unnecessary.
In the meantime, in order to split a high frequency signal and to enable two signals after splitting to have a phase difference of 180 degrees, a rat race circuit is generally used. The rat race circuit splits a signal by branching a signal line into two lines and provides the two signal lines after the branching with a length difference corresponding to a ½ wavelength of a signal frequency to be transmitted, thereby enabling the two split signals to have a phase difference of 180 degrees.
However, the line length corresponding to a ½ wavelength of a signal frequency is about several mm or several cm even for a high frequency signal of GHz or more and requires a large circuit area. Due to this, it is difficult to incorporate the rat race circuit into the microwave IC.
Hence, instead of obtaining a phase difference by using a difference of the line lengths, a method has been suggested in which a phase difference of 180 degrees is obtained by using a balun circuit that converts a non-differential transmission line such as CPW line or micro strip line into a differential transmission line such as slot line or CPS (Coplanar Strips) line, or a differential transmission line into a non-differential transmission line (for example, Yasuhiro Hamada, Kenichi Maruhashi, Masaharu Ito, Shuya Kishimoto, Takao Morimoto, and Keiichi Ohata, “A60-GHz-band Compact IQ Modulator MMIC for Ultra-high-speed Wireless Communication,” 2006 IEEE MTT-S International Microwave Symposium Digest, pp. 1701-1704, June 2006 (Non-Patent Document 1)).
As shown in FIG. 2, a balun circuit described in Non-Patent Document 1 has first CPW line 61, second CPW line 62a and third CPW line 62b, which are signal input/output ports, FCPW (Finite Ground Coplanar Waveguide) line 63, first CPS line 65a and second CPS line 65b, which are differential transmission lines, FCPW-CPW conversion section 64 that converts first CPW line 61 into FCPW line 63, FCPW-CPS conversion branch section 66 that converts FCPW line 63 into first CPS line 65a and second CPS line 65b, first CPS-CPW conversion section 67a that converts first CPS line 65a into second CPW line 62a and second CPS-CPW conversion section 67b that converts second CPS line 65b into third CPW line 62b, which are formed on substrate 69.
First CPW line 61, second CPW line 62a, third CPW line 62b and FCPW line 63 are non-differential transmission lines having a central conductor and two ground conductors arranged to sandwich the central conductor therebetween. The two ground conductors of first CPW line 61, second CPW line 62a, third CPW line 62b and FCPW line 63 are connected by air bridges 68, respectively.
In the balun circuit shown in FIG. 2, first CPW line 61 is converted into FCPW line 63 by CPW-FCPW conversion section 64 and FCPW line 63 is branched and converted into first CPS line 65a and second CPS line 65b by FCPW-CPS conversion branch section 66. In addition, first CPS line 65a is converted into second CPW line 62a by first CPS-CPW conversion section 67a and second CPS line 65b is converted into third CPW line 62b by second CPS-CPW conversion section 67b. Here, the central conductor of second CPW line 62a is connected to the ground conductor of FCPW line 63 and the central conductor of third CPW line 62b is connected to the central conductor of FCPW line 63. In addition, the ground conductor of second CPW line 62a is connected to the central conductor of FCPW line 63 and the ground conductor of third CPW line 62b is connected to the ground conductor of FCPW line 63.
Like this, the relation between the connection of the central and ground connectors of second CPW line 62a to the central and ground connectors of FCPW line 63, and the connection of the central and ground connectors of third CPW line 62b to the central and ground connectors of FCPW line 63 is reversed. Thus, when a signal is inputted from first CPW line 61, differential signals having a phase difference of 180 degrees are outputted from second CPW line 62a and third CPW line 62b. Since such structure does not use a method that obtains a phase difference of 180 degrees by an electrical length, it is possible to appropriately shorten the length of the CPS line and to make a circuit size small. Further, since the ground conductors of the respective CPW lines are connected to each other, the ground potential of each CPW line is same and the above structure can be easily applied to an integrated circuit. By connecting the ground conductors, the phase difference of the signals outputted from second CPW line 62a and third CPW line 62b is not always 180 degrees. However, it is possible to compensate for a deviation of the phase difference by making the lengths of first CPS line 65a and second CPS line 65b different.
However, the balun circuit of the Non-Patent Document 1 has the following problems.
A first problem is that when the balun circuit is formed on a conductive substrate made of silicon, for example, the insertion loss of the balun circuit is increased. This is caused by a substrate loss. That is, this occurs because the electromagnetic fields occurring in the CPW, FCPW and CPS lines are spread into the substrate and a signal is attenuated by a resistance component of the substrate. Hence, in high frequency lines formed on a conductive substrate, a conductive layer referred to as a ground shield is generally arranged in an underlying layer, which is connected to a ground potential to shield an electric field and thus to prevent a loss by the substrate. However, even when the ground shield is applied to the balun circuit, the power is not equally split and a phase difference is not 180 degrees. In other words, it is impossible to operate as a balun circuit.
This is because the coupling between the ground shield and each of two strip-shaped conductors constituting the CPS line is predominant over the coupling between the strip-shaped conductors, so that a micro strip line mode becomes a main transmission mode and the CPS line section does not resultantly operate a differential transmission line.
A second problem is that it is difficult to reduce the circuit size.
This is caused by the CPS lines of the balun circuit. To be more specific, since the CPS lines are such that the two strip-shaped conductors are arranged in a line, the CPS lines occupy an area obtained by adding at least a gap of the conductors and a conductor width corresponding to two conductors. Furthermore, since the spread of the electromagnetic fields in the horizontal direction is large, it is necessary to keep another circuit including the ground conductors at a distance.